1. Field of the Invention
The invention relates to a suction gripper for transferring the trailing edge of a sheet in a turning device of a sheet-fed rotary printing machine.
Published, Non-Prosecuted German Patent Application DE 38 29 626 A discloses a sheet-fed rotary printing machine having a turning device in which, during perfecting operation, a suction gripper is pivoted out of a periphery of a cylinder disposed downstream of a back-pressure cylinder toward the circumferential surface of the back-pressure cylinder. There, it grips the trailing edge of the sheet to be turned and, before passing the gripper center line in between the two cylinders, guides the trailing edge into the periphery of the downstream cylinder. Because of the rigid and stiff configuration of the suction gripper and of the fixedly predefined pivoting path, during the processing of printed materials of different thicknesses, the result is sharply varying pressure forces with which the suction head of the suction gripper is pressed against the paper and, respectively, the circumferential surface of the back-pressure cylinder when picking up the sheet trailing edge by suction. Thus, in the case of very thin papers, such as bible paper, there is the risk that the suction head of the suction gripper will not touch the surface of the sheet at all. This can result in relatively large fluctuations in the location of the trailing edge of the sheet in relation to the suction gripper, and lead to register errors when transferring the sheet trailing edge to subsequent gripper devices. However, during the processing of thick printed materials, high mechanical forces are exerted on the trailing edge of the sheet, on the suction gripper as such and on the gear mechanism actuating the suction gripper. As a result of which pressure points may form on the sheet and the suction gripper or the gear mechanism is subjected to high mechanical stress and hence to high wear.
Published, Non-Prosecuted German Patent Application DE 41 06 703 A1 discloses a swinging sucker system, which can be pivoted about a first rocker shaft and, in perfecting operation, grips the trailing edge of a sheet to be transferred in the region of a gripper center line between upstream and downstream sheet-conveying cylinders. In order to adapt to different printed material thicknesses, the swinging sucker system is mounted on a rocker, which can be pivoted by an eccentric about a second rocker shaft. In order to limit the maximum adjustment travel of the eccentric and hence the maximum printed material thickness to be printed, the rocker is provided with a stop. The stop, in the case of maximum sheet thicknesses to be processed, rests on an associated stop on the cylinder body of the downstream sheet-conveying cylinder. Also disposed between the rocker and the cylinder body is a compression spring which, in order to produce a counterforce when the eccentric is rotated in the direction of the maximum printed material thickness, exerts a counterforce on the rocker. Apart from the fact that the device described is mechanically complicated to produce and contains numerous moving parts, it does not permit any automatic adaptation to different printed material thicknesses.
German Democratic Republic Patent No. 142 953 discloses a sucker in a turning device of a sheet-fed rotary printing machine, which has a suction head which is produced from flexible material and automatically adapts to printed materials of different thicknesses. Because of the flexible configuration of the suction head, in the event of the sheet being tautened before the transfer to a downstream gripper device, positional displacements of the sheet occur, which rule out the in-register transfer of the sheet trailing edge. Furthermore, the suction head made of flexible material that is described in the document has the disadvantage that its elastic properties can change sharply over the course of time and that, in the case of processing very thick sheets, for example those with a thickness in the region of 1 mm, very high forces nevertheless act on the sucker as such. These forces being brought about by the fact that it is not just the thin-walled lip of the sucker but the entire rubber body of the same that is compressed. This contradicts the requirement to configure the rubber lip to be as thin-walled as possible in the case of processing very thin sheets, in order to rule out the local sucking in and tearing of the paper in the region of the suction heads. The flexible suction heads described therefore constitute only a compromise.